False Memories Implanted in Mice

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Tinkering with the brains of mice, scientists have given the
rodents memories of events that never occurred.

The researchers used a technique that involves activating neurons
with light to train mice to "remember" a painful experience in a
completely different context from that in which they experienced
the pain. The
false memories were encoded by brain cells in the same way as
real memories are sealed in.

Even without any scientific manipulation, memories can be
unreliable. Many studies have shown the limits of eyewitness
testimony in the courtroom, for instance. But few studies have
looked at how false memories are formed at the cellular level.
[ 5 Wild
Facts About Your Memory ]

"In humans, false memory phenomena are very well established, and
in some cases it can have had serious legal consequences," said
study researcher Susumu Tonegawa, a neuroscientist at MIT in
Cambridge, Mass.

When
the brain forms a memory, a population of brain cells is
thought to undergo lasting physical or chemical changes, creating
what's called a "memory engram." Memory has two phases: First,
the memory is acquired by activating these brain cells, and later
it is recalled by reactivating these cells. Scientists had
hypothesized, but never proved, these memory cells existed.

Implanting mice memories

Last year, Tonegawa and his colleagues showed
that such cells do exist in a part of the hippocampus, the
brain’s memory center. The researchers genetically engineered
mice to make certain neurons light-sensitive — a technique known
as optogenetics — so that shining a blue light on the cells
activated them.

The mice were put in a chamber where they experienced foot
shocks, causing them to freeze in fear. The animals learned to
associate the shocks with the chamber, forming
a fear memory. Then the researchers put the mice in a
different chamber, and shone a blue light on the cells that
encoded the foot-shock memory. The animals reacted as fearfully
as if they were in the first chamber.

In the present study, Tonegawa's group took the experiment a step
further. First they allowed the mice to explore the first chamber
without getting a foot shock. Then they put the mice in a second
chamber where they gave them foot shocks while shining a blue
light on the cells that encoded the memory of the first chamber.
They wanted to see whether, when they put the mice back into the
first chamber, they would react as if they had been shocked
there.

The mice did exactly that, showing fear when they were placed in
the first chamber, even though they had never experienced a shock
there. The researchers had succeeded in implanting a false memory
into the mice. The findings were detailed online today (July 25)
in the journal Science.

"Memory comes from experience," Tonegawa told LiveScience. But in
this case, the animal never experienced any fear in the first
chamber, and yet the animal was fearful of that chamber, he said.

False human memory

The findings provide a model for how false memories might be
formed in humans. Before the advent of DNA testing, many
criminals were convicted primarily on the basis of
eyewitness testimony. When their DNA was tested later, "three
out of four people imprisoned for many years based primarily on
witness recall turned out be innocent," Tonegawa said.

Tonegawa described the famous case of a woman who was watching TV
when a man broke into her apartment and raped her. The man she
accused as her rapist was a psychiatrist who had been on TV at
the time she was raped. The psychiatrist was in a TV studio and
therefore could not have been the rapist, and yet the woman swore
it was him, because she had formed a false memory associating the
sound of his voice with the rape.

So how could humans have evolved the ability to form false
memories? Tonegawa speculates that false memory is the price
humans pay for creativity. Our imagination makes us inventive,
but it also makes us susceptible to conflating events that did
and didn’t happen.

"Humans are very creative," he said. "As a byproduct, we form
false memories."